Carbon fiber reinforced composites (CFRC) are being used to an increasing extent throughout the technological arts. These composites are quickly becoming the material of choice in a wide range of aerospace applications. Their superb strength to weight ratios not only portend use throughout aviation in structural panels, but suggest use in applications such as rocket nozzles on missiles and spacecraft. Likewise, development in submersibles and other pressure structures are turning to these CFRC.
Knowledge of the electrical resistivity of CFRC is necessary to determine their antennae characteristics; to determine their shielding properties; to accurately interpret defect indications in their nondestructive inspection; and to provide quality control during fabrication. A method to interpret defect indications and measure the depth of a defect is taught in an application for U.S. Pat. Ser. No. 294,621 filed January 9th, 1988 by Applicants.
The individual fiber and matrix material components that make up such a composite are often of nonuniform quality and moreover are laminated in different and varied patterns. Additionally, fabricators can be expected to make occasional human errors in the number and spacing of plies in the layup. Slight changes in curing cycles and rates of heating can also have major effects on mechanical properties, but no obvious effect in the visual appearance of the finished parts. In general, the deviation in properties for composite materials on multiple tests is far greater than considered acceptable for established metal technology. Refined and improved nondestructive testing apparatus and methods must be developed to allow inspection to establish, with greater assurance, the exact quality of the part under surveillance.
There are a number of methods available to measure the electrical resistivity (or electrical conductivity) of materials, but all have shortcomings with respect to carbon fiber composite materials. The various contact methods for measuring resistivity are not appropriate for many of these materials because the insulating properties of the matrix material in some classes of CFRC prevent good electrical contact. The existence of coatings also impede measurement in many applications.
Commercially available conductivity meters are designed to measure the nonfrequency dependent conductivity of metals. As a consequence they do not provide conductivity values as a function of frequency and they are applicable only to relatively high conductivities (&gt;1.4% of the International Annealed Copper Standard (IACS)).
Noncontact, through-transmission eddy current methods can provide the necessary measurements, but they are available only in the laboratory and require access to both surfaces of the material. Both surfaces are not always accessible.
Many methods used to measure electrical resistivity require calibration standards. These resistivity standards in the high range of resistivities are not available. The disclosed device and techniques do not require standards.
Collins et al., U.S. Pat. No. 4,747,310 filed May 31, 1988 and Collins et al., U.S. Pat. No. 4,745,809 filed May 24, 1988, use eddy current techniques along with ultrasound and magnetic induction to measure relative conductivity, but teach no method to measure absolute conductivity in thick metal sections or carbon fiber reinforced composites. A knowledge of absolute conductivity is necessary to insure through thickness eddy current inspections; to measure defect depth and to evaluate electromagnetic shielding properties.
The inability to measure the resistivity of very high resistivity materials without providing good electrical contact to the test material, and without providing access to both sides of the test material, is generally recognized in the art. The method and the instrument disclosed herein provide these capabilities.
Therefore, it is an object of this invention to teach a method and device of nondestructive, noncontact measurement of actual (as opposed to relative) electrical resistivity.
It is another object of the instant invention to disclose a method for the measurement of the resistivity of materials whose resistivity is so high that calibration standards do not exist.
It is yet another object of the present invention to teach a method for the measurement of the resistivity of carbon fiber reinforced composites.
It is another object of the invention to teach a method for the measurement of the resistivity of graphite epoxy materials where the epoxy prevents the forming of good electrical contact.
It is yet another object of the present invention to provide a method for the measurement of the electrical resistivity of components when only one surface is accessible.
It is another object to teach a method for detecting variations in fiber density which cause variations in electrical resistivity.
It is another object of the instant invention to teach a method for the measurement of the electrical resistivity of a material over a range of frequencies when the resistivity of the test material is frequency dependent.
It is a further object of the invention to teach a method and instrument which can estimate the resistivity of conducting materials over an unlimited range of resistivities.
It is still another object of the present invention to teach a device for measuring actual resistivity of carbon fiber reinforced composites.
It is still another object of the instant invention to teach a device which exhibits the required frequency and sensitivity to measure resistivity in materials having 50,000 microhm.cm.
It is yet another object of the present invention to provide a device for measuring absolute resistivity having an accuracy over a frequency range adequate to employ the hereinbelow methods.
Other objects, advantages and novel features of the invention will appear from a reading of the following detailed description of the invention when considered in conjunction with the accompanying drawings, wherein: